In situ NMR to monitor bulk photopolymerization kinetics

The ability to precisely measure photopolymerization kinetics is paramount to controlling curing characteristics and material properties in photocurable systems. However, traditional methods to measure kinetics, such as Fourier- transform infrared spectroscopy (FTIR), are often limited by broadening mechanisms intrinsic to the system and poor spectral resolution. In this work, we present an in situ NMR spectroscopy technique to monitor bulk photopolymerization reactions wherein the polymer system is separated from the locking solvent via a capillary insert and photoexcited using an LED-coupled fiber optic. This technique allows for the purity of the reaction to be preserved while also benefiting from the high spectral resolution and rich chemical information offered by NMR. Rate constants (kp’) and ultimate monomer conversion are determined for four systems: (a) neat hexyl acrylate, (b) isobornyl methacrylate, (c) N,N-dimethylacrylamide, and (d) hexyl acrylate in the presence of a crosslinking species. By observing kinetic data of simple photopolymer systems, this work demonstrates the utility of in situ NMR photopolymerization as a complementary technique to conventional FTIR for the kinetic monitoring of photopolymer materials.